1. Field of the Invention
The present invention relates to an over-current detection circuit and method for a power-supply device. In particular, it relates to an over-current detection circuit and method for a power-supply device, whereby, when detecting an over-current using the ON-voltage of a main MOSFET, variations of the ON-voltage due to process variations of the ON-resistance of the main MOSFET or the influence of switching noise is reduced, thereby stabilizing the detection accuracy.
2. Background Art
Conventionally, a built-in power supply of an LSI circuit such as an FPGA or CPU chip is suitable for a compact or low-cost system/unit due to reduction of external components and for high-speed current response (high di/dt) of a load side. As an over-current detection method in such case, the ON-voltage of a main MOSFET is used. The following methods are known as such a method using the ON-voltage of a main MOSFET.
According to an example of the method using the ON-voltage of a main MOSFET, an over-current is detected by comparing the ON-voltage of a main MOSFET obtained via a sample-and-hold circuit and a reference voltage for over-current setting in a comparator (see JP Patent Publication (Kokai) No. 2003-60449 A, for example).
According to another example of the method using the ON-voltage of a main MOSFET, a sense MOSFET with a size ratio of n:1 with respect to a main MOSFET is provided, and a current that is 1/n of an over-current value is caused to flow through the sense MOSFET. When these two MOSFETs are turned on at the same time, the ON-voltage of the main MOSFET and the ON-voltage of the sense MOSFET are compared in a comparator, so as to detect an over-voltage (see U.S. Pat. No. 6,031,361, for example).